This application is a competitive renewal of R01 MH 53608-09. The focus of the proposal is to examine the function of the immediate early gene (lEG) termed Arc, and its contribution to activity-dependent plasticity. Arc was cloned in our laboratory based on its rapid induction in response to neuronal activity (Lyford, 1995). Arc mRNA is strikingly induced during learning behaviors and its transcriptional response provides the basis for a novel imaging method that detects stable neural networks in brain (Guzowski, 1999). Arc protein appears to be essential for learning and memory as interruption of Arc induction blocks the maintenance phase of LTP and disrupts memory (Guzowski, 2000). These studies focus attention on the molecular basis of Arc protein function at the synapse. In preliminary studies for the present renewal, we find that Arc protein interacts with certain SH3 domain proteins and also interacts with CaMKII. Moreover, Arc expression induces a LTD-like down regulation of AMPAR responses in hippocampal neurons. Aim 1 will examine the hypothesis that Arc functions to recruit the SH3 domain protein termed endophilin 3, together with CaMKII, to form a unique endocytic vesicle that is involved in AMPAR trafficking. Biochemical studies will define the composition of the putative Arc endosome, and structural determinants of Arc essential for its action. Aim 2 will examine the mechanism of Arc function at the excitatory synapse. Preliminary studies indicate that the SH3 interaction site of Arc is required to evoke down regulate AMPAR, and proposed studies will test the hypothesis that the Arc-endosome selectively modifies trafficking of distinct AMPAR. We will also examine mechanisms that regulate localized expression of Arc protein. Aim 3 will generate mouse transgenic models to test the contribution of the SH3 and CaMKII interaction domains of Arc on synaptic and behavioral plasticity. These studies will identify novel mechanisms that underlie long-term, activity-dependent neuronal plasticity, and explore promising links with the biology of disease.